1. Field of the Invention
The present invention relates to an apparatus for detecting the distribution of radioactive sources over a predefined area and more particularly, to an apparatus comprising a two dimensional avalanche counter and a position encoder for precisely ascertaining the frequency of occurrence and location of particles or rays generated by radioactive sources in a structural configuration that is particularly useful for screening recombinant DNA.
2. Prior Art
A recombinant DNA is a synthetic DNA molecule containing genes from two or more different organisms. In recent years recombinant DNA has become an important tool in genetic engineering. The use of recombinant DNA permits many copies of a desired genetic region to be replicated thereby permitting analysis of gene arrangement by molecular techniques. Typically, the process involved includes the production of molecular clones by introducing recombinant DNAs into bacteria, usually a bacterial virus commonly referred to as phage or bacteriophage. The molecular clones produced in this fashion are then typically analyzed for those that obtain the desired gene or genes. In order to isolate bacterial clones to be analyzed. The recombinant DNA bearing bacteria is screened for the desired DNA. In this process the bacterial clones to be analyzed are replicated so that analysis does not destroy the clone. The bacteria can then be lysed and their DNA liberated. Typically, the DNA is liberated directly onto a nitrocellulose filter and then made radioactive by hybridizing radioactive RNA or complementary DNA to the DNAs on the filter. The filters are then rinsed making them ready for DNA location and isolation by a process called autoradiography. It is to this process known as autoradiography, namely, a process for locating radioactive DNAs on a filter, that the present invention is particularly directed.
Generally speaking, prior art autoradiography has relied upon the radioactive effect in creating an image on photographic film or X-ray film. Unfortunately, such prior art methods require that the film be exposed for very long periods of time such as days or even weeks in order to produce a visualization of the distribution and amounts of radioactively-labelled molecules. Such lengthy periods required to produce autoradiographs utilizing X-ray or photographic film, can be extremely disadvantageous and costly. As a result, a number of alternative faster techniques, including some borrowed from the nuclear particle physics art, have been considered for use in the DNA screening process for autoradiographic location of radioactive DNAs. However such prior art devices are either too cumbersome, too costly, cover too small an area or lack adequate spatial resolution. Furthermore, such prior art devices often must be operated at extremely high voltages using exotic gases at very high pressures. By way of example, a number of pertinent devices are disclosed in the following patents:
U.S. Pat. No. 3,717,766, Allard et al; PA1 U.S. Pat. No. 3,461,293, Horowitz; PA1 U.S. Pat. No. 3,449,573, Lansiart et al; PA1 U.S. Pat. No. 3,975,639, Allemand; PA1 U.S. Pat. No. 3,373,283, Lansiart et al;
Other relevant prior art has been disclosed in the parent application, Ser. No. 370,333 filed on Apr. 21, 1983 and that prior art discussion is hereby incorporated by reference into the present application.